What Are Those Big Jellyfish Fossils Doing in Wisconsin?

It's rare to find a jellyfish fossil--not having a skeleton, they easily decay.
So why is an entire horde of them preserved in central Wisconsin?

Whitey Hagadorn from Caltech wondered the same thing after he learned about this exclusive deposit in the Upper Cambrian Mt. Simon-Wonewoc Sandstone in central Wisconsin. During the Cambrian, Wisconsin enjoyed a tropical environment and was covered by a shallow inland sea. Hagadorn and his colleagues present their hypothesis on how these jellyfish were preserved in the February issue of GEOLOGY.

They believe that the jellyfish were preserved because of a lack of erosion from sea water and wind, the lack of scavengers, and the lack of any significant sediment disturbance by other organisms after the jellyfish were stranded in the sand.

"It is very rare to discover a deposit which contains an entire stranding event of jellyfish," Hagadorn said. "These jellyfish are not just large for the Cambrian, but are the largest jellyfish in the entire fossil record. What is also of interest is that they were among the largest two types of predators in the Cambrian."

He thinks jellyfish may have been under-appreciated in previous studies of Cambrian ecosystems and that they were probably important predators in Cambrian food chains.

He's also documenting the deposit and trying to figure out how it was formed.

"We use fossils to assess the diversity and ecology of ancient communities," Hagadorn said. "To date, most of our information about the trophic (food chain) structure of the Cambrian--when multicellular animals burst onto the scene--is based on animals with hard parts or on exceptional deposits which contain soft-bodied organisms. Despite the widespread study of rocks from this interval, very few jellyfish are known, and even fewer large pelagic (free-swimming) animals are known. So that means that when we analyze the trophic structure of the Cambrian--who ate whom, who ate them, and so forth, or when we analyze how abundant each type of organism was in each part of the food chain, we may have been inadvertently omitting a huge amount of information about all of the soft-bodied animals that were swimming around in the water column, munching on other organisms, but which were rarely fossilized. This deposit provides us a rare opportunity to study such animals."